To prove that Doc has invented it, Marty has Doc drive him out to the place where he's hidden the DeLorean. Doc is overly delighted when he compares the drawing he made of a flux capacitor and finds the real thing installed on the DeLorean. After returning to Doc's estate, they manage to plug in Marty's "portable television studio" to see the video Marty had filmed in 1985. Doc becomes ecstatic when they reach the point on tape where he will say that time travel requires 1.21 gigawatts of energy from the plutonium to power the flux capacitor.
Doc explains that plutonium is very hard to come by in 1955 and that the only energy source capable of that amount of power is a lightning bolt. Predicting the strike zone of a lightning bolt is impossible and Doc tells Marty he may be stuck forever in 1955. But then Marty remembers the flyer the woman gave him about the lightning bolt that is going to strike the clock tower at 10:04 PM next Saturday night and hands it to Doc. Now that he knows a date, Doc begins working on a plan to harness the power of the bolt and send Marty home.
When Marty says he can hang out in 1955 for a week, Doc objects, warning him that it could be detrimental to future history and jeopardize his entire existence. He asks Marty if he had any interaction with anyone in the last few hours and Marty drops the bomb about preventing the first meeting between his father and mother. Doc asks to see the photo of Marty and his siblings again. Dave's head is now completely gone. This means that in order to go back to 1985, Marty first needs to make his parents fall in love and have their first kiss within a week.
Doc takes Marty to the high school the next morning. Marty is amazed to find that there is no graffiti on the building, unlike in 1985. After peeping through a classroom window and watching Lorraine cheating on a test, they spot George in the hallway during a passing period, seeing him being picked on (in part because of the large "KICK ME" note taped to his back). George is further demoralized when Strickland (who in 1955 is down to his last dregs of hair) appears and tells him he is a slacker.
Doc is baffled that Lorraine could fall in love with someone like George, and Marty admits that his best guess is she originally felt sorry for him after her dad ran over him. Doc recognizes their relationship as a version of the Florence Nightingale effect, which happens when doctors develop romantic feelings for their patients. Marty tries encouraging George to talk to Lorraine, however, an attempt to simply introduce them to each other fails because Lorraine is already smitten with Marty.
Doc finds that the situation is more serious than they'd thought. George lacks the self-confidence to ask Lorraine out, as he fears that he couldn't handle a rejection if she said "no", and getting them together permanently could be impossible. Over lunch, Marty tries again to convince George by saying Lorraine has been talking about him and that he should ask her to the Enchantment Under the Sea dance. George spends his lunch by himself writing science fiction short stories. Marty asks to read one of them and George refuses, saying he's afraid people would be critical.
He also suggests that Lorraine may want to go with someone else to the dance, namely Biff, who we see is across the cafeteria, sitting with Lorraine and trying to grope her. Marty immediately marches over to them and pulls Biff off his mother. Biff begins pushing Marty, however, Marty, unlike his meek father, begins pushing back and is about to fight Biff when Strickland breaks it up. Marty follows George home and begins pleading with George to ask Lorraine out. George continues to refuse and tells Marty that no one in the world will make him change his mind.
That night, Marty breaks into George's room in his radiation suit, places his Walkman headphones on George and gives him a blast of ear-splitting Eddie Van Halen guitar riffing. Marty claims he is Darth Vader, an extraterrestrial from the planet Vulcan, and intimidates George into asking Lorraine out, threatening him with a "brain melting gun" (actually a hairdryer). To keep George from calling for his parents, Marty chloroforms him, before jumping out the window and into Doc's car.
The next day, George rushes up to Marty at the Texaco station, disheveled and frantic, having overslept, while Marty is trying to open a Pepsi. George knows he needs to ask Lorraine out but he doesn't know what he should say.
Marty takes George back to Lou's diner, where Lorraine is hanging out with her friends. Marty suggests to George that he tell Lorraine, "Destiny has brought me to you." George orders a chocolate milkshake to calm his nerves before approaching Lorraine. It gets off to a shaky start when, in a fit of nervousness, George accidentally mangles the lines Marty gave him. George's attempt comes to a grinding halt when Biff and his friends come in to toss him out.
As Biff demands money from George, Marty, sitting at the counter, "accidentally" trips Biff. Biff turns his anger on Marty, and is about to punch him when Marty tricks him into looking away, giving Marty the opportunity to shove Biff and bolt out the door. Once outside, Marty grabs a passing girl's scooter, tears off the crate and turns the bottom into a skateboard. Biff and his goons chase Marty in Biff's car around the town square. Watching all this, Lorraine is all smiles and love struck starry eyed.